1. Field of the Invention
A waste heat boiler successive to a suspension smelting furnace usually is a horizontal boiler consisting of two parts, the radiation section and the convection section. The purpose of the radiation section is to cool the gases, so that the molten particles contained therein are solidified, and the temperature sinks down below the sintering temperature of the particles prior to conducting the gases to the convection section of the boiler. This prevents their sintering in the convection section, in the cooling pipework whereby the final heat of the gases is recovered.
The temperature of the gases coming from suspension smelting is in the range of 1,300.degree. C., and their dust content is fairly high. This brings forth difficulties already in the radiation section. After reacting unfavourably with oxygen, the dust sticks onto the surface of the steam pipes provided in the wall, and consequently, owing to the formed insulating layer, weakens heat transfer from the gas to the pressurized vapor flowing in the pipes. In the worst case, the formed dust layer may grow so thick that it comes off and causes danger when falling down. If the heat transfer in the radiation section weakens, the gases flow too hot into the pipework of the convection section, causing even worse blocking problems there. Blocking may also take place in the aperture in between the uptake shaft of the flash smelting furnace and the boiler, which in known fashion leads to changes in the circulations and pressures, and to successive interruptions in the production.
2. Description of the Prior Art
In the prior art there is known a rapping system for removing dust accretions in a waste heat boiler. Rapping is known to have had a positive effect, but only in eliminating the symptoms, not the cause of the disturbance. When this method is applied too vigorously, the drawbacks are soon apparent in the reduction of the service life of the boiler equipment. There is also known a method for feeding oxygen-bearing gas into the radiation section in order to bring about advantageous reactions.
Another prior art method is the use of separate cooling panels arranged in the radiation section parallel to the boiler flows in order to improve heat recovery; now the flow is free to proceed on both sides of the panel. In similar fashion, in the radiation section there has been used an intermediate cooling wall transversal to the direction of the flow, by means of which wall the main flow is made to proceed from underneath the wall and thus lengthen effective delay time; the area of cooling surface is naturally expanded. Experiences from transversal walls are, however, bad, because they easily tend to accumulate dust accretions.
In some cases the convection section is located on a level lower than the radiation section in order to change the gas flow pattern. The purpose is to prevent the gas from flowing directly along the ceiling of the radiation section of the waste heat boiler, so that the convection section is placed lower down than the radiation section, in which case the rear part of the ceiling in the radiation section is downwardly inclined. The U.S. Pat No. 4,530,311 introduces a method for descending the radiation section in a stepwise fashion and for thus preventing a direct flow into the convection section. At the same time, the radiation section is provided with cooling panels parallel to the flow, in order to increase the heat transfer surface. These work well, if they are correctly designed.
However, dust accretions accumulate in horizontal tunnel-type waste heat boiler constructions owing to the high dust-content of the gases formed in suspension smelting; these accretions are an obstruction for an effective operation of the waste heat boiler, as well as for the whole suspension smelting process. These operation difficulties may cause production breaks in suspension smelting processes and thus result in great economical losses for the producer.
The tendency to dust accretions is enhanced for instance by the following factors:
In the radiation section of the waste heat boiler, only the roof and top parts of the walls are in effective use--presupposing that they are clean. Because a major part of the heat load is focused on a small area of the boiler, it is difficult to keep the waste heat boiler clean. Moreover, the hot dust-bearing gases flow partly uncooled directly into the convection section of the waste heat boiler, which causes the molten dust particles to stick onto the cooling pipework, and the cooled particles to sinter.
The formation of dust accretions is also enhanced when an increased number of dust particles collides with the boiler walls. The collision probability is higher for all walls that are located transversally to the flow. In addition to this, the bottom part of the waste heat boiler is a poor receiver of radiation, but allows a harmfully long delay time (causing eddy currents) for part of the dust-bearing gases and forms good conditions for the formation of harmful sulfur trioxide. The formation of SO.sub.3 may, because of the humidity contained in the gases, and/or because of possible boiler leaks, form sulfuric acid, which corrodes the constructions of the equipment. It is also pointed out that the dust accretion difficulties in a waste heat boiler increase as the boiler size grows.
In these prior art cases, the aim has been the removal of dust accretions (rapping devices), the prevention thereof (additional air supply), an extended delay time (transversal panel walls, stepwise descended radiation section, lowered convection section) or an expanded heat transfer surface (panel walls). However, in all these methods, the aperture in between the furnace and the boiler remains a favourable ground for dust accretions.
The purpose of the waste heat boiler construction of the present invention is to eliminate the drawbacks of the above described prior art constructions and to achieve a waste heat boiler that is more effective and more secure in operation, the said boiler being suited to cooling dust-bearing gases formed in the suspension smelting process and at the same time to dust recovery. Moreover, the structure according to the present invention is extremely well suited to raising the capacity of old boilers. The invention also relates to a corresponding method for forming a bulk-like flow and to lengthen the delay time of the whole flow in the radiation section of the boiler.
According to the invention, the radiation section is divided into two flow parts by means of a vertical intermediate plate, i.e. panel wall, suspended downwardly from the ceiling and made of steam pipes, or by means of a wall construction composed of several mainly successive panel walls. The wall or wall construction is arranged essentially in the lengthwise direction of the radiation section, but so that the gas inlet and outlet openings are provided on different sides of the wall. Underneath, the wall is open for the flow. The panel wall structure of the invention forms a space for the inlet flow from the radiation section, and the transversal flow area of this space is gradually reduced in the first part, i.e. in the inlet part, and respectively expanded in the second part, i.e. the outlet part. In the rear end the main flow, in order to maintain its rate, consequently fills the space also further down. Because the flow cannot get out directly at the end of the first part of the convection section, it turns, from underneath the wall according to the present invention, to the outlet part, and flows then along the bottom part of this half first back towards the furnace, but has no access back to the inlet opening, because the wall of the invention prevents it. Instead of this, because the outlet part in turn gradually expands towards its end (towards the convection section), the flow rises and turns to flow, again in a plug-like formation, towards the inlet of the convection section. This ensures that the cold flow does not return to the outlet opening of the furnace uptake shaft. At the same time, the delay time favourable for heat transfer is lengthened.
Apart from sulfur dioxide, the dusts flowing to a waste heat boiler from a suspension smelting furnace often contain oxides or sulfides of the metal to be smelted, such as copper or nickel. Particularly sulfides easily form dust accretions in the boiler, where they stick onto the cooled surfaces and form sulfates while oxidizing there. It has been proved that if dust-containing gases are mildly oxidized while the gas temperature is over 600.degree. C., the metal compounds of the dusts react forming sulfates, but in the temperature range 450.degree.-600.degree. C., the primary reaction is the oxidizing of sulfur dioxide to sulfur trioxide, which should be avoided in order to prevent the formation of sulfuric acid. It is favourable for the process that the dusts react to sulfates, in case the sulfatizing can be carried out so that the sulfates do not form dust accretions. By employing the method of the present invention, the formation of dust accretions is prevented so that the gases are kept in a uniform bulk flow without small, separate eddy flows. The whole bulk flow is oxidized, and thus the oxidizing takes place in a uniform fashion, at a high temperature, so that the sticking of the dusts to the surfaces is prevented.
The present invention relates to a waste heat boiler construction successive to a smelting furnace, such as a suspension smelting furnace, wherein the total space of the waste heat boiler is advantageously made use of, and simultaneously the direct flowing of the dust-bearing gases radiation section of the waste heat boiler to the convection section thereof is prevented in order to reduce the tendency to dust accretions caused by these gases.